1. Field of the Invention
The present invention relates to solid-state imaging devices, and more specifically to a solid-state imaging device for color imaging in which a pixel is formed using an island-shaped semiconductor (a pillar-shaped semiconductor), with a high pixel density, a high sensitivity, and a wide dynamic range being achieved.
2. Description of the Related Art
Solid-state imaging devices for color imaging, such as CCD and CMOS imaging devices, are generally used in video cameras, still cameras, and so forth. In these uses, solid-state imaging devices for color imaging are required to have improved performance such as a high pixel density, a high sensitivity, and a wide dynamic range.
FIG. 6 illustrates a cross-sectional structural diagram of a CMOS color solid-state imaging device of the related art (see, for example, U.S. Patent Application Publication No. 2005/0082627).
Isolation silicon oxide layers (hereinafter, represented by SiO2 layers) 101a to 101d are formed on a surface of a P-region (hereinafter, a P-type semiconductor region including acceptor impurities is represented by a “P region”) silicon (hereinafter represented by “Si”) substrate 100 using, for example, the LOCOS (Local Oxidation of Silicon) method. N regions (hereinafter, an N-type semiconductor region including donor impurities is represented by an “N region”) 102a to 102c are formed between the isolation SiO2 layers 101a to 101d. 
In FIG. 6, the P-region substrate 100 and the N regions 102a to 102c form photodiodes based on P-N junctions. Incident light (electromagnetic energy waves) incident on the upper surfaces of the N regions 102a to 102c undergoes photoelectric conversion in the N regions 102a to 102c and the P-region substrate 100 located below the N regions 102a to 102c, and signal charges (in this case, free electrons) are generated. The generated signal charges are accumulated in the photodiodes, and are extracted to an external output circuit as the signal output current in a certain period.
An interlayer SiO2 layer 103 is formed on the isolation SiO2 layers 101a to 101d and the N regions 102a to 102c. Metal wirings 104a to 104d are formed on the interlayer SiO2 layer 103. Further, a protective insulating layer 105 composed of, for example, a SiO2 or organic material layer is formed on the interlayer SiO2 layer 103 and the metal wirings 104a to 104d. 
In FIG. 6, the upper surface of the protective insulating layer 105 is planarized. A color filter 106R for red (R), a color filter 106G for green (G), and a color filter 106B for blue (B) are arranged on or above the protective insulating layer 105 so as to surround the N regions 102a to 102c, which form photodiodes, when viewed in a direction perpendicular to the upper surface. Isolation insulating layers 107a to 107c are formed between the color filter 106R for red (R), the color filter 106G for green (G), and the color filter 106B for blue (B).
The color filter 106R for red (R) is a layer that transmits mainly red wavelength light in incident light incident on the upper surfaces of the N regions 102a to 102c. The color filter 106G for green (G) is a layer that transmits mainly green wavelength light in the incident light. The color filter 106B for blue (B) is a layer that transmits mainly blue wavelength light in the incident light.
The color filter 106R for red (R) (hereinafter, the “color filter 106R” for short), the color filter 106G for green (G) (hereinafter, the “color filter 106G” for short), and the color filter 106B for blue (B) (hereinafter, the “color filter 106B” for short) are formed by a photoresist containing pigments or dyes. The color filter 106G for green (G) is formed using photolithography technology, and is coated with the protective insulating layer 105 and is also coated with the isolation insulating layer 107a from above the color filter 106G. The color filter 106R and the color filter 106B are formed on or over the isolation insulating layer 107a using photolithography technology. The isolation insulating layer 107c also functions as a protection layer of the color filters 106R, 106G, and 106B.
The isolation SiO2 layers 101a to 101d, the N regions 102a to 102c, and the metal wirings 104a to 104d formed on or over the P-region substrate 100 illustrated in FIG. 6 are formed using advanced CMOS micromachining technology, which is a micromachining technology used for microprocessors, memories, and so forth. Although not illustrated in FIG. 6, CMOS transistors for forming a driver circuit, a signal processing circuit, and so forth are formed on or over the P-region substrate 100 using the advanced CMOS micromachining technology in a manner similar to that for the isolation SiO2 layers 101a to 101d, the N regions 102a to 102c, and the metal wirings 104a to 104d. 
The advanced CMOS micromachining technology is not available for the formation of the color filter 106R, the color filter 106G, and the color filter 106B. The color filter 106R, the color filter 106G, and the color filter 106B are formed using photolithography technology that utilizes a different photoresist material.
The micromachinable dimensions (minimum machining dimensions) of the color filter 106R, the color filter 106G, and the color filter 106B which are realized by the photolithography technology are greater (larger) than the micromachinable dimensions which are realized by the advanced CMOS micromachining technology described above. In this manner, the inability to apply micromachining technology to the color filters 106R, 106G, and 106B hinders a further increase in the pixel density of CMOS solid-state imaging devices.
In addition, the process and apparatus for forming the color filters 106R, 106G, and 106B are different from the process and apparatus used in advanced CMOS micromachining technology described above, which may cause an increase in cost. This imposes a problem of reduction in the cost of solid-state imaging devices.
Additionally, the material of the color filters 106R, 106G, and 106B causes light absorption, which thus prevents the color filter 1068 from achieving a light transmittance of 100% in the red (R) wavelength region. Similarly to this, the color filter 106G and the color filter 106B are also prevented from achieving a light transmittance of 100% in the green (G) wavelength region and the blue (B) wavelength region, respectively. In this manner, light absorption which prevents improvements in the light transmittance of the color filters 106R, 106G, and 106B hinders the sensitivity of CMOS color solid-state imaging devices from increasing.
Hereinafter, another solid-state imaging device for color imaging of the related art will be described with reference to FIGS. 7A to 7C.
FIG. 7A illustrates a cross-sectional structural diagram of a solid-state imaging device in which one island-shaped semiconductor constitutes one pixel (see, for example, U.S. Patent Application Publication No. 2010/0200731).
Referring to FIG. 7A, a signal line N+ region 112 (hereinafter, a semiconductor region including a large number of donor impurities is referred to as an “N+ region”) is formed on a substrate 111. An island-shaped semiconductor 110 is formed on the signal line N+ region 112. In the island-shaped semiconductor 110, an insulating layer 114 is formed on an outer periphery of a P region 113 adjoining the signal line N+ region 112, and a conductive layer 115 is further formed with the insulating layer 114 interposed between the conductive layer 115 and the P region 113. An N region 116 is formed on the outer periphery of the P region 113 above the conductive layer 115. A P+ region (hereinafter, a semiconductor region including a large number of acceptor impurities is referred to as a “P+ region”) 117 is formed on the N region 116 and the P region 113. The P+ region 117 is connected to a pixel selection line conductive layer 118. The insulating layer 114 described above is continuously formed in such a manner as to surround an outer periphery of the island-shaped semiconductor 110. Similarly to the insulating layer 114, the conductive layer 115 is also continuously formed in such a manner as to surround the outer periphery of the island-shaped semiconductor 110.
In this solid-state imaging device, the P region 113 and the N region 116 in the island-shaped semiconductor 110 form a photodiode region 119. When light is incident on the P+ region 117 side on a top layer of an upper end of the island-shaped semiconductor 110, signal charges (here, free electrons) are generated in a photoelectric conversion region in the photodiode region 119. The generated signal charges are accumulated mainly in the N region 116 of the photodiode region 119.
The island-shaped semiconductor 110 further has a junction field-effect transistor having a gate which is defined by the N region 116, a source which is defined by the P+ region 117, and a drain which is defined by the P region 113 in the vicinity of the signal line N+ region 112.
In this solid-state imaging device, the drain-source current (output signal) of the junction field-effect transistor changes in accordance with the quantity of signal charges accumulated in the N region 116, and is extracted from the signal line N+ region 112 as the signal output.
The island-shaped semiconductor 110 further has a MOS transistor having a source which is defined by the N region 116 of the photodiode region 119, a gate which is defined by the conductive layer 115, a drain which is defined by the signal line N+ region 112, and a channel which is defined by the P region 113 between the N region 116 and the signal line N+ region 112.
In this solid-state imaging device, the signal charges accumulated in the N region 116 are discharged to the signal line N+ region 112 by the application of the ON voltage (high-level voltage) to the conductive layer 115 that is the gate of the MOS transistor.
Herein, the term “high-level voltage” refers to a positive voltage having a high level in a case where the signal charges are free electrons, and the term “low-level voltage” refers to a voltage whose absolute value is lower than the “high-level voltage”. Accordingly, if the signal charges are positive holes, the “high-level voltage” implies a negative voltage having a low level, and the “low-level voltage” refers to a voltage that is closer to 0 V than the “high-level voltage”.
The imaging operation of this solid-state imaging device includes the operations (1) to (3) below while the ground voltage (=0 V) is applied to the signal line N+ region 112, the conductive layer 115, and the P+ region 117:
(1) A signal charge accumulating operation for accumulating in the N region 116 the signal charges generated in the photoelectric conversion region (the photodiode region 119) by the incidence of light on the top surface of the upper end of the island-shaped semiconductor 110,
(2) A signal charge read operation for reading as the signal current the source-drain current of the junction field-effect transistor that has been modulated by the potential of the N region 116 which has changed in accordance with the quantity of accumulated signal charges while the ground voltage is applied to the signal line N+ region 112 and the conductive layer 115 and a positive voltage is applied to the P+ region 117, and
(3) A reset operation for discharging the signal charges accumulated in the N region 116 to the signal line N+ region 112 while the ground voltage is applied to the P+ region 117 and a positive voltage is applied to the conductive layer 115 and the signal line N+ region 112 after the signal charge read operation.
FIG. 7B illustrates a plan view of a solid-state imaging device of the related art in which island-shaped semiconductors P11 to P33 (each of which corresponds to the island-shaped semiconductor 110 in FIG. 7A) constituting pixels are arranged in a two-dimensional matrix. The island-shaped semiconductors P11 to P33 constituting pixels are formed on signal line N+ regions 112a, 112b, and 112c (each of which corresponds to the signal line N+ region 112 in FIG. 7A).
In the island-shaped semiconductors P11 to P33, pixel selection line conductive layers 118a, 118b, and 118c (each of which corresponds to the pixel selection line conductive layer 118 in FIG. 7A) are each continuously formed in each row extending in the horizontal direction. Similarly to this, in the island-shaped semiconductors P11 to P33 constituting pixels, conductive layers 115a, 115b, and 115c (each of which corresponds to the conductive layer 115 in FIG. 7A) are each continuously formed in each row extending in the horizontal direction.
Color filters B1, B2, and B3 for blue (B), color filters R1, R2, and R3 for red (R), and color filters G1, G2, and G3 for green (G) are formed on the island-shaped semiconductors P11 to P33.
With the configuration described above, the blue (B) signal current is obtained from the pixels of the island-shaped semiconductors P11, P21, and P31, the green (G) signal current is obtained from the pixels of the island-shaped semiconductors P12, P22, and P32, and the red (R) signal current is obtained from the pixels of the island-shaped semiconductors P13, P23, and P33.
In the configuration illustrated in FIG. 7B, spaces are required between the island-shaped semiconductors P11 to P33 in order to ensure a mask alignment margin in manufacturing, which is necessary to form a mask so as to surround the color filters B1, B2, B3, R1, R2, R3, G1, G2, and G3 and the island-shaped semiconductors P11 to P33. These spaces limit an increase in the density of the pixels. In this solid-state imaging device of the related art, furthermore, as in the solid-state imaging device illustrated in FIG. 6, light absorption in the color filters B1, B2, B3, R1, R2, R3, G1, G2, and G3 prevents an increase in sensitivity.
FIG. 7C illustrates a cross-sectional structural diagram taken along the line C-C′ in FIG. 7B.
Referring to FIG. 7C, the signal line N+ regions 112a, 112b, and 112c are formed on a substrate 111a, and the island-shaped semiconductors P11, P12, and P13 are formed on the signal line N+ regions 112a, 112b, and 112c. Insulating layers 120a are formed on the substrate 111a between the island-shaped semiconductors P11, P12, and P13, and the conductive layer 115a is formed on outer peripheries of P regions 113a, 113b, and 113c adjoining the signal line N+ regions 112a, 112b, and 112c of the island-shaped semiconductor P11, P12, and P13 with insulating layers 114a, 114b, and 114c disposed between the conductive layer 115a and the P regions 113a, 113b, and 113c. The conductive layer 115a is formed so as to connect the island-shaped semiconductors P11, P12, and P13 to one another, and N regions 116a, 116b, and 116c of photodiodes are formed on the outer peripheries of the island-shaped semiconductor P11, P12, and P13 above upper ends of the conductive layer 115a, which is located inside the island-shaped semiconductor P11, P12, and P13. Insulating layers 120b are formed on or over the conductive layer 115a and the insulating layers 120a between the island-shaped semiconductors P11, P12, and P13, and P+ regions 117a, 117b, and 117c are formed on top layers of upper ends of the island-shaped semiconductors P11, P12, and P13. The pixel selection line conductive layer 118a is formed on the insulating layers 120b so as to adjoin the P+ regions 117a, 117b, and 117c. An insulating layer 120c is formed on or over the insulating layers 120b, the pixel selection line conductive layer 118a, and the P+ regions 117a, 117b, and 117c. The top surface of the insulating layer 120c is planarized, and the color filter B1 for blue (B), the color filter G1 for green (G), and the color filter R1 for red (R) are formed on the insulating layer 120c. An overcoat insulating layer 120d is formed on the color filters B1, G1, and R1 and the insulating layer 120c. The color filters B1, G1, and R1 are formed using a manufacturing method similar to that for the solid-state imaging device illustrated in FIG. 6, unlike the micromachining technology used for the pixel structure of the island-shaped semiconductors P11, P12, and P13. Thus, there are problems in increasing the density of pixels and reducing cost.
Hereinafter, another solid-state imaging device of the related art which is capable of color imaging without using color filters will be described with reference to FIGS. 8A to 8D.
FIG. 8A illustrates a cross-sectional structural diagram of this solid-state imaging device (see, for example, U.S. Patent Application Publication No. 2012/104478).
Referring to FIG. 8A, an N region (N well) 122 is formed in a P-region substrate 121, and a P region (P well) 123 is formed in the N region 122. Here, an N region 124 is formed in the P region 123. A diode formed of the P-region substrate 121 and the N region 122, a diode formed of the N region 122 and the P region 123, and a diode formed of the P region 123 and the N region 124 are reversely biased.
Here, the depth of the N region 124 from the top surface of the P-region substrate 121 is desirably approximately 0.2 μm, the depth of the P region 123 from the top surface of the P-region substrate 121 is desirably approximately 0.6 jam, and the depth of the N region 122 from the top surface of the P-region substrate 121 is desirably approximately 2 μm.
Of the incident light incident on the top surface of the P-region substrate 121, mainly blue (B) wavelength light undergoes photoelectric conversion in a diode region 126a (in FIG. 8A, a region surrounded by a dotted line) that is formed of the P region 123 and the N region 124, and generated signal charges are accumulated in the diode region 126a. Mainly green (G) wavelength light undergoes photoelectric conversion in a diode region 126b (in FIG. 8A, a region surrounded by a dotted line) that is formed of the P region 123 and the N region 122, and generated signal charges are accumulated in the diode region 126b. Further, mainly red (R) wavelength light undergoes photoelectric conversion in a diode region 126c (in FIG. 8A, a region surrounded by a dotted line) that is formed of the P-region substrate 121 and the N region 122, and generated signal charges are accumulated in the diode region 126b. 
Then, the signal charges accumulated in the diode region 126a are read as a blue (B) signal using an ammeter 125a, and the signal charges accumulated in the diode region 126a are read as a green (G) signal using an ammeter 125b, and the signal charges accumulated in the diode region 126c are read as a red (R) signal using an ammeter 125c. 
The solid-state imaging device illustrated in FIG. 8A is capable of color imaging without using color filters because it utilizes the light absorption characteristics of the semiconductors (made of silicon (Si) in this case) illustrated in FIG. 8B.
As illustrated in FIG. 8B, a large proportion of the blue (B) wavelength light (λ=400 nm) is absorbed in the vicinity of the Si (silicon) surface, and light having a larger wavelength (λ) such as green (G) wavelength light (λ=550 nm) and red (R) wavelength light (λ=700 nm) penetrates further inside the Si (silicon), and is absorbed. Accordingly, mainly a blue (B) signal is obtained in the diode region 126a, mainly a green (G) signal is obtained in the diode region 126b, and mainly a red (R) signal is obtained in the diode region 126c. 
FIG. 8C illustrates the light wavelength (λ) dependence of the outputs obtained from the ammeters 125a, 125b, and 125c. 
As illustrated in FIG. 8C, the blue (B) signal output VB obtained from the ammeter 125a mainly has the blue (B) wavelength light output component, the green (G) signal output VG obtained from the ammeter 125b mainly has the green (G) wavelength light output component, and the red (R) signal output VR obtained from the ammeter 125c mainly has the red (R) wavelength light output component. A signal computation process such as white balance is performed on the signal outputs VB, VG, and VR to obtain desired RGB signals.
FIG. 8D illustrates a plan view of the cross-sectional structure illustrated in FIG. 8A, when viewed from above the P-region substrate 121, and also illustrates output circuits.
As illustrated in FIG. 8D, the N region (N well) 122 is formed in the P-region substrate 121, and the P region (P well) 123 is formed in the N region 122. The N region 124 is formed in the P region 123. A contact hole 127a, a contact hole 127b, and a contact hole 127c are formed in the N region 124, the P region 123, and the N region 122, which are on the same surface as the top surface of the P-region substrate 121, respectively.
The N region 124 and an output circuit 129a (in FIG. 8D, a region surrounded by a dotted line) are connected via the contact hole 127a and a lead 128a connecting with the contact hole 127a. The P region 123 and an output circuit 129b (in FIG. 8D, a region surrounded by a dotted line) are connected via the contact hole 127b and a lead 128b connecting with the contact hole 127b. The N region 122 and an output circuit 129c (in FIG. 8D, a region surrounded by a dotted line) are connected via the contact hole 127c and a lead 128c connecting with the contact hole 127c. 
Each of the output circuits 129a, 129b, and 129c is constituted by an amplifier MOS transistor Am configured to sense the voltage of the corresponding one of the N region 124, the P region 123, and the N region 122, a row selection MOS transistor RS, and a reset MOS transistor Re for discharging the signal charges accumulated in the corresponding one of the diode regions 126a, 126b, and 126c. A blue (B) signal is read from a signal line 130a for blue (B), a green (G) signal is read from a signal line 130b for green (G), and a red (R) signal is read from a signal line 130c for red (R). The output circuits 129a, 129b, and 129c are formed on a surface of the P-region substrate 121 outside the N region 122.
The solid-state imaging device illustrated in FIGS. 8A and 8D has features in that, compared to the solid-state imaging device illustrated in FIG. 6, the solid-state imaging device is capable of color imaging without using color filters and the diode regions 126a, 126b, and 126c where photoelectric conversion is performed on RGB wavelength light overlap one another in the depth direction.
In this solid-state imaging device, however, since the N region 124 that receives blue (B) wavelength light, which is required to have the largest light-receiving area, is formed inside the P region 123 and the N region 122, a large pixel size is used to obtain desired blue (B) wavelength light sensitivity. There is another problem in that the solid-state imaging device is not capable of obtaining signals of white (W) wavelength light including all of blue (B) wavelength light, green (G) wavelength light, and red (R) wavelength light.
In the solid-state imaging device illustrated in FIG. 6, a new pixel having none of the color filters 106B, 106G, and 106R is formed separately from the pixels having the color filters 106B, 106G, and 106R, and a white (W) wavelength light signal is obtained from the pixel, thereby achieving a high sensitivity or a wide dynamic range (see, for example, H. Honda, Y. Iida, Y. Egawa, H. Seki; “A Color CMOS Imager with 4×4 White-RGB Color Filter Array for Increased Low-Illumination Signal-to-Noise Ratio”, III Transaction on Electron Devices, Vol. 56, No. 11, pp. 2398-2402 (2009); and Y. Egawa, N. Tanaka, N. Kawai, H. Seki, A. Nakano, H. Honda, Y. Iida, M. Monoi: “A White-RGB CFA-Patterned CMOS Image Sensor with Wide Dynamic Range”, ISSCC 2008, Digest of Technical Papers, pp. 52-53 (2008)).
This technology utilizes the ability of white (W) pixels to read larger signal current than RGB pixels, thereby obtaining a high SN ratio (signal/noise ratio). The solid-state imaging device illustrated in FIG. 8A, in contrast, is not capable of directly reading the signal current of white (W) wavelength light.
In the solid-state imaging devices of the related art illustrated in FIGS. 6 and 7C, the pixel structure formed below the color filters 106B, 106G, 106R, B1, G1, and R1 is formed using advanced CMOS micromachining technology.
However, the CMOS micromachining technology is not available for the formation of the color filters 106B, 106G, 106R, B1, G1, and R1. The color filters 106B, 106G, 106R, B 1, G1, and R1 are formed using photolithography technology that utilizes a different photoresist material.
Therefore, the micromachining dimensions of the color filters 106B, 106G, 106R, B1, G1, and R1 are greater (larger) than the micromachining dimensions which are realized by the advanced CMOS micromachining technology. The micromachining technology for the color filters 106B, 106G, 106R, B1, G1, and R1 thus limits a further increase in the pixel density of CMOS solid-state imaging devices.
In addition, the process and apparatus for forming the color filters 106B, 106G, 106R, B1, G1, and R1 are different from the process and apparatus used in advanced CMOS micromachining technology, which may cause an increase in cost. This imposes a problem of reduction in the cost of solid-state imaging devices.
Additionally, in the solid-state imaging devices of the related art illustrated in FIGS. 6 and 7C, the material of the R, G, and B color filters 106R, 106G, 106B, B1, G1, and R1 causes light absorption, which thus prevents the color filters 106R and R1 from achieving a light transmittance of 100% in the red (R) wavelength region. Similarly to this, the color filters 106G and G1 and the color filters 106B and B1 for blue (B) are also prevented from achieving a light transmittance of 100% in the green (G) wavelength region and the blue (B) wavelength region, respectively. In this manner, light absorption which prevents improvements in the light transmittance of the color filters 106R, 106G, 106B, B 1, G1, and R1 hinders the sensitivity of CMOS color solid-state imaging devices from increasing.
In the solid-state imaging device of the related art illustrated in FIG. 8A, furthermore, the diode regions 126a, 126b, and 126c where photoelectric conversion is performed on RGB wavelength light are formed so as to overlap one another in the depth direction, and the N region 124 that receives blue (B) wavelength light, which is required to have the largest light-receiving area, is formed inside the P region 123 and the N region 122. Thus, a large pixel size is used to obtain desired blue (B) wavelength light sensitivity. There is another difficulty in that the solid-state imaging device is not capable of directly obtaining signals of white (W) wavelength light including all of blue (B) wavelength light, green (G) wavelength light, and red (R) wavelength light, and it is difficult to achieve a high sensitivity and a wide dynamic range utilizing white (W) wavelength light signals.